The present invention relates to a modulation device, which is designed to receive an input signal comprising a succession of pulses with a predetermined width, and to produce an output signal comprising a succession of pulses with an adjustable width.
A device of this type is known from Japanese patent application published under number JP 01202916. The modulation device which is described in this application is designed to receive an input signal with an asymmetrical nature, and is provided with a first and a second branch, which make it possible to decrease or increase respectively the width of the pulses of the output signal of the device. For this purpose, the said branches comprise an assortment of delay cells and logic gates, a selection of which of the branches must be activated being made possible by means of a multiplexer.
The known modulation device has a complex structure, which has a certain number of disadvantages.
Firstly, it is inevitable that each of the logic gates included in this device induces a delay in the signals which pass through them. Although delays of this type can be ignored or disregarded in applications in which the signals to be processed have a frequency which is low or not very high, for example of approximately a KiloHertz or MegaHertz, these delays, which are typically approximately a nanosecond, or a tenth of a nanosecond in the present state of the art, will have a detrimental effect on processing of signals which have a very high frequency, for example of approximately a GigaHertz.
Moreover, the delay cells which are used in the known device must be perfectly paired to enable deliberate absence of modulation of the pulse width of the output signal, in which situation the two branches must introduce alternately the same phase shift in the output signal. Pairing of this type is very difficult, and therefore very costly, to implement in practice.
In addition, it is often preferable, particularly when the frequency of the signals to be processed is very high, to use differential structures which have better rates of rejection in common mode, and factors of immunity to noise which are higher than those of the asymmetrical structures such as that of the known device. For this purpose, the above described structure must be duplicated, which will increase further still the complexity of the known device, when configured in differential mode, and, consequently, its production cost, while making a modulation device of this type even more sensitive to the aforementioned disadvantages.
The object of the invention is to eliminate these disadvantages, by proposing a modulation device which has a simple structure, without logic gates and delay cells, and which permits processing of very high-frequency differential signals.
In fact, according to the invention, a modulation device according to the introductory paragraph includes:
a first and a second transistor, which are fitted as a differential pair, the conduction of which is controlled by means of the input signal of the device;
a capacitive element, which is provided with a first and a second terminal, which are connected respectively between the first and second transistor;
adjusting means for adjusting a potential of at least one of the said terminals; and
comparing means which are designed to supply the output signal of the device, an instantaneous value of which is representative of the sign of a voltage, which is taken from the terminals of the capacitive element.
In the device according to the invention, quasi-simultaneous transitions which delimit the pulses of the input signal generate gradients in curves which are representative of the development of the potentials of the terminals of the capacitive element, owing to the progressive charges and discharges of the latter. Each intersection of said potential curves corresponds to a transition of the output signal of the modulation device. By giving rise to offsetting of the potential of at least one of the terminals of the capacitive element, the adjusting means make it possible to offset over a period of time the instants at which intersections of this type occur, and thus to adjust the width of the pulses of the output signal of the device. The processing of the input signal is thus substantially analog in the device according to the invention, which provides it with great speed, and makes it particularly suitable for xe2x80x9cvery high-frequencyxe2x80x9d applications.
In addition, the device according to the invention has a very simple, and substantially symmetrical structure, which allows it to process differential signals, and provides it with a high rate of rejection in common mode, and with a factor of immunity to high levels of noise.
In a particular embodiment of the invention, the adjusting means include means for decreasing, and means for increasing potential, which are associated with at least one of the terminals of the capacitive element.
This embodiment makes it possible to control simply and flexibly the intersections of the potential curves, and thus the width of the pulses of the output signals of the modulation device according to the invention.
According to a particularly advantageous embodiment of the invention, means for decreasing and means for increasing potential will be associated with each of the terminals of the capacitive element, which will permit greater flexibility and better resolution of the adjustment of the width of the pulses of the modulation device according to the invention.
As previously stated, the modulation device according to the invention is particularly well suited for processing signals which have a very high frequency. It will thus advantageously be used for controlling very high-rate data emitters, such as emitters of light signals in optical telecommunications systems, which have throughputs which are measured in Gigabits/second. The invention thus relates to a light signal emitter, which includes a source which is designed to emit radiation when a control signal for said source has an active state, the emitter additionally including a modulation device as previously described, for adjusting the duration of the active states of the control signal.
In one of its most advantageous applications, the invention also relates to a telecommunications system including:
an emitter for light signals, as previously described;
a receiver for light signals; and
a propagation medium, which permits transit of said signals from the emitter to the receiver.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter